This research plan describes two studies that test different properties of the neurohormone melatonin in humans undergoing a large shift in their sleep-wake schedule. The first is a field study to determine whether appropriately-timed melatonin administration can help shift the internal circadian pacemaker to adjust to a 9-hr shift in the sleep-wake schedule. In this protocol, 48 subjects will undergo a 9-hr advance or delay in their sleep-wake schedule for 8 days. Subjects will be randomly assigned to take placebo or melatonin on the first 4 days of the shifted schedule. Phase will be measured using continuous recording of core body temperature. In the second study, we will test melatonin's efficacy in improving sleep during the day after night work without shifting endogenous circadian phase. Subjects (N=10) will each participate in a melatonin trial and a placebo trial. After a week of baseline sleep, subjects will come to the lab for 2 nights of simulated night work followed by daytime sleep. Sleep will be polysomnographically-recorded on 2 baseline nights, 2 day sleep episodes, and l recovery night. In addition, subjects will undergo Multiple Sleep Latency Tests and performance tests during the night work episodes.